Management of head and media dimensional stability

ABSTRACT

An apparatus, according to one embodiment, includes a magnetic head having servo readers of known pitch, a drive mechanism for passing a magnetic recording tape over the magnetic head, and a controller electrically coupled to the magnetic head. The controller is configured to measure, using signals from the magnetic head, a servo band difference at various locations along a length of the magnetic recording tape. The controller is also configured to cause storage of servo band difference information about the servo band difference measurements in association with the magnetic recording tape.

BACKGROUND

The present invention relates to data storage systems, and moreparticularly, this invention relates to managing dimensional stabilityissues.

In magnetic storage systems, magnetic transducers read data from andwrite data onto magnetic recording media. Data is written on themagnetic recording media by moving a magnetic recording transducer to aposition over the media where the data is to be stored. The magneticrecording transducer then generates a magnetic field, which encodes thedata into the magnetic media. Data is read from the media by similarlypositioning the magnetic read transducer and then sensing the magneticfield of the magnetic media. Read and write operations may beindependently synchronized with the movement of the media to ensure thatthe data can be read from and written to the desired location on themedia.

An important and continuing goal in the data storage industry is that ofincreasing the density of data stored on a medium. For tape storagesystems, that goal has led to increasing the track and linear bitdensity on recording tape, and decreasing the thickness of the magnetictape medium. However, the development of small footprint, higherperformance tape drive systems has created various challenges rangingfrom the design of tape head assemblies for use in such systems todealing with tape dimensional instability.

Tape drives write and read multiple data tracks simultaneously. It iscritical that all data tracks are written in the correct locations forproper operation during subsequent readback. If the dimension of thehead changes due to temperature or other causes, or if the transducerson the head are not positioned in the proper, design-specified locationsdue to fabrication variations, then data tracks will be written/read atincorrect locations. Likewise, if the media is not consistent in itsdimensions, then the data tracks will move after writing and not be inthe same location when the tape is read. In either case, successful readback of the data will be impaired.

In the past, the management of dimensional stability issues was done bytolerance control. Each component had limits on how much variation fromdesign parameters was allowed. As track density increased, the allowablelimits for variation were also decreased. However, to continue toincrease track density to support high tape cartridge capacities, thismethod is no longer feasible, as components cannot be made at lowervariation. Accordingly, the capacity growth of future tape storageschemes will be limited if new techniques for managing head and mediadimensional stability are not developed.

SUMMARY

An apparatus, according to one embodiment, includes a magnetic headhaving servo readers of known pitch, a drive mechanism for passing amagnetic recording tape over the magnetic head, and a controllerelectrically coupled to the magnetic head. The controller is configuredto measure, using signals from the magnetic head, a servo banddifference at various locations along a length of the magnetic recordingtape. The controller is also configured to cause storage of servo banddifference information about the servo band difference measurements inassociation with the magnetic recording tape.

Other aspects and embodiments of the present invention will becomeapparent from the following detailed description, which, when taken inconjunction with the drawings, illustrate by way of example theprinciples of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic diagram of a simplified tape drive systemaccording to one embodiment.

FIG. 1B is a schematic diagram of a tape cartridge according to oneembodiment.

FIG. 2A illustrates a side view of a flat-lapped, bi-directional,two-module magnetic tape head according to one embodiment.

FIG. 2B is a tape bearing surface view taken from Line 2B of FIG. 2A.

FIG. 2C is a detailed view taken from Circle 2C of FIG. 2B.

FIG. 2D is a detailed view of a partial tape bearing surface of a pairof modules.

FIG. 3 is a partial tape bearing surface view of a magnetic head havinga write-read-write configuration.

FIG. 4 is a partial tape bearing surface view of a magnetic head havinga read-write-read configuration.

FIG. 5 is a side view of a magnetic tape head with three modulesaccording to one embodiment where the modules all generally lie alongabout parallel planes.

FIG. 6 is a side view of a magnetic tape head with three modules in atangent (angled) configuration.

FIG. 7 is a side view of a magnetic tape head with three modules in anoverwrap configuration.

FIGS. 8A-8C are schematics depicting the principles of tape tenting.

FIG. 9 is a representational diagram of files and indexes stored on amagnetic tape according to one embodiment.

FIG. 10 is a representational diagram of the effect of tape lateralexpansion and consequential transducer misregistration.

FIG. 11 is a chart exemplifying the increase in error rate toward outerends of a reader array due to a misregistration of the readers towardthe outer ends of the array.

FIG. 12 is a flow chart of a process for characterizing a magneticrecording tape of a tape cartridge.

FIG. 13 is a flowchart of a process for characterizing a magneticrecording tape of a tape cartridge in one illustrative approach.

FIG. 14 illustrates exemplary sample SBD reference values from Beginningof Tape (BOT) to End of Tape (EOT) gathered during performance of theprocess of FIG. 12.

FIG. 15 is a flowchart of a process for controlling writing to amagnetic recording tape of a tape cartridge.

FIG. 16 is a flowchart of a process for one exemplary mode of use duringwriting.

FIG. 17 is a flowchart of a process for controlling writing to amagnetic recording tape of a tape cartridge.

FIG. 18 is a flowchart of a process for one exemplary mode of use duringreading.

FIG. 19 is a flowchart of a process for characterizing the present stateof a tape of a tape cartridge relative to an earlier state thereof.

DETAILED DESCRIPTION

The following description is made for the purpose of illustrating thegeneral principles of the present invention and is not meant to limitthe inventive concepts claimed herein. Further, particular featuresdescribed herein can be used in combination with other describedfeatures in each of the various possible combinations and permutations.

Unless otherwise specifically defined herein, all terms are to be giventheir broadest possible interpretation including meanings implied fromthe specification as well as meanings understood by those skilled in theart and/or as defined in dictionaries, treatises, etc.

It must also be noted that, as used in the specification and theappended claims, the singular forms “a,” “an” and “the” include pluralreferents unless otherwise specified.

The following description discloses several preferred embodiments ofmagnetic storage systems, as well as operation and/or component partsthereof.

A method for characterizing a magnetic recording tape of a tapecartridge, according to one approach, includes measuring, using amagnetic head having servo readers of known pitch, a servo banddifference at various locations along a length of a magnetic recordingtape of a tape cartridge. The servo band difference measurements and/orderivatives thereof are stored in association with the tape cartridge.This procedure creates a characterization of the magnetic recording tapethat is useful for assessing aging of the magnetic recording tape, aswell as improving subsequent reading and writing operations.

A method for controlling writing to a magnetic recording tape of a tapecartridge, according to another approach, includes retrieving servo banddifference information from a tape cartridge. A servo band difference ismeasured at various locations along a length of a magnetic recordingtape of the tape cartridge using servo readers of a magnetic head. Theservo band difference measurements and/or derivatives thereof arecompared to the retrieved servo band difference information, and awriting operation is controlled based at least in part on a result ofthe comparing. As tape and head dimensional changes affect the locationof written tracks, it is critical that the current writing operationdoes not excessively trim, or narrow, the previously written datatracks, thereby rendering the data tracks unreadable. The foregoingprocedure, among other things, may be used to prevent overwriting of thepreviously-written tracks during shingling.

A method for controlling reading of a magnetic recording tape of a tapecartridge, according to yet another approach, includes retrieving servoband difference information about a tape cartridge. A servo banddifference at various locations along a length of a magnetic recordingtape of the tape cartridge is measured using servo readers of a magnetichead. The servo band difference measurements and/or derivatives thereofare compared to the retrieved servo band difference information. Areading operation s controlled based at least in part on a result of thecomparing. For example, this procedure may improve reading performance,e.g., by assisting in centering the reader transducers (also referred toherein as readers) as much as possible above the appropriate data track.

FIG. 1A illustrates a simplified tape drive 100 of a tape-based datastorage system, which may be employed in the context of the presentinvention. While one specific implementation of a tape drive is shown inFIG. 1A, it should be noted that the embodiments described herein may beimplemented in the context of any type of tape drive system.

As shown, a tape supply cartridge 120 and a take-up reel 121 areprovided to support a tape 122. One or more of the reels may form partof a removable cartridge and are not necessarily part of the tape drive100. The tape drive, such as that illustrated in FIG. 1A, may furtherinclude drive motor(s) to drive the tape supply cartridge 120 and thetake-up reel 121 to move the tape 122 over a tape head 126 of any type.Such head may include an array of readers, writers, or both.

Guides 125 guide the tape 122 across the tape head 126. Such tape head126 is in turn coupled to a controller 128 via a cable 130. Thecontroller 128, may be or include a processor and/or any logic forcontrolling any subsystem of the drive 100. For example, the controller128 typically controls head functions such as servo following, datawriting, data reading, etc. The controller 128 may include at least oneservo channel and at least one data channel, each of which include dataflow processing logic configured to process and/or store information tobe written to and/or read from the tape 122. The controller 128 mayoperate under logic known in the art, as well as any logic disclosedherein, and thus may be considered as a processor for any of thedescriptions of tape drives included herein, in various embodiments. Thecontroller 128 may be coupled to a memory 136 of any known type, whichmay store instructions executable by the controller 128. Moreover, thecontroller 128 may be configured and/or programmable to perform orcontrol some or all of the methodology presented herein. Thus, thecontroller 128 may be considered to be configured to perform variousoperations by way of logic programmed into one or more chips, modules,and/or blocks; software, firmware, and/or other instructions beingavailable to one or more processors; etc., and combinations thereof.

The cable 130 may include read/write circuits to transmit data to thetape head 126 to be recorded on the tape 122 and to receive data read bythe tape head 126 from the tape 122. An actuator 132 controls positionof the tape head 126 relative to the tape 122.

An interface 134 may also be provided for communication between the tapedrive 100 and a host (internal or external) to send and receive the dataand for controlling the operation of the tape drive 100 andcommunicating the status of the tape drive 100 to the host, all as willbe understood by those of skill in the art.

FIG. 1B illustrates an exemplary tape cartridge 150 according to oneembodiment. Such tape cartridge 150 may be used with a system such asthat shown in FIG. 1A. As shown, the tape cartridge 150 includes ahousing 152, a tape 122 in the housing 152, and a nonvolatile memory 156coupled to the housing 152. In some approaches, the nonvolatile memory156 may be embedded inside the housing 152, as shown in FIG. 1B. In moreapproaches, the nonvolatile memory 156 may be attached to the inside oroutside of the housing 152 without modification of the housing 152. Forexample, the nonvolatile memory may be embedded in a self-adhesive label154. In one preferred embodiment, the nonvolatile memory 156 may be aFlash memory device, read-only memory (ROM) device, etc., embedded intoor coupled to the inside or outside of the tape cartridge 150. Thenonvolatile memory is accessible by the tape drive and the tapeoperating software (the driver software), and/or another device.

By way of example, FIG. 2A illustrates a side view of a flat-lapped,bi-directional, two-module magnetic tape head 200 which may beimplemented in the context of the present invention. As shown, the headincludes a pair of bases 202, each equipped with a module 204, and fixedat a small angle α with respect to each other. The bases may be“U-beams” that are adhesively coupled together. Each module 204 includesa substrate 204A and a closure 204B with a thin film portion, commonlyreferred to as a “gap” in which the readers and/or writers 206 areformed. In use, a tape 208 is moved over the modules 204 along a media(tape) bearing surface 209 in the manner shown for reading and writingdata on the tape 208 using the readers and writers. The wrap angle θ ofthe tape 208 at edges going onto and exiting the flat media supportsurfaces 209 are usually between about 0.1 degree and about 3 degrees.

The substrates 204A are typically constructed of a wear resistantmaterial, such as a ceramic. The closures 204B may be made of the sameor similar ceramic as the substrates 204A.

The readers and writers may be arranged in a piggyback or mergedconfiguration. An illustrative piggybacked configuration comprises a(magnetically inductive) writer transducer on top of (or below) a(magnetically shielded) reader transducer (e.g., a magnetoresistivereader, etc.), wherein the poles of the writer and the shields of thereader are generally separated. An illustrative merged configurationcomprises one reader shield in the same physical layer as one writerpole (hence, “merged”). The readers and writers may also be arranged inan interleaved configuration. Alternatively, each array of channels maybe readers or writers only. Any of these arrays may contain one or moreservo track readers for reading servo data on the medium.

FIG. 2B illustrates the tape bearing surface 209 of one of the modules204 taken from Line 2B of FIG. 2A. A representative tape 208 is shown indashed lines. The module 204 is preferably long enough to be able tosupport the tape as the head steps between data bands.

In this example, the tape 208 includes 4 to 32 data bands, e.g., with 16data bands and 17 servo tracks 210, as shown in FIG. 2B on a one-halfinch wide tape 208. The data bands are defined between servo tracks 210.Each data band may include a number of data tracks, for example 1024data tracks (not shown). During read/write operations, the readersand/or writers 206 are positioned to specific track positions within oneof the data bands. Outer readers, sometimes called servo readers, readthe servo tracks 210. The servo signals are in turn used to keep thereaders and/or writers 206 aligned with a particular set of tracksduring the read/write operations.

FIG. 2C depicts a plurality of readers and/or writers 206 formed in agap 218 on the module 204 in Circle 2C of FIG. 2B. As shown, the arrayof readers and writers 206 includes, for example, 16 writers 214, 16readers 216 and two servo readers 212, though the number of elements mayvary. Illustrative embodiments include 8, 16, 32, 40, and 64 activereaders and/or writers 206 per array, and alternatively interleaveddesigns having odd numbers of reader or writers such as 17, 25, 33, etc.An illustrative embodiment includes 32 readers per array and/or 32writers per array, where the actual number of transducer elements couldbe greater, e.g., 33, 34, etc. This allows the tape to travel moreslowly, thereby reducing speed-induced tracking and mechanicaldifficulties and/or execute fewer “wraps” to fill or read the tape.While the readers and writers may be arranged in a piggybackconfiguration as shown in FIG. 2C, the readers 216 and writers 214 mayalso be arranged in an interleaved configuration. Alternatively, eacharray of readers and/or writers 206 may be readers or writers only, andthe arrays may contain one or more servo readers 212. As noted byconsidering FIGS. 2A and 2B-2C together, each module 204 may include acomplementary set of readers and/or writers 206 for such things asbi-directional reading and writing, read-while-write capability,backward compatibility, etc.

FIG. 2D shows a partial tape bearing surface view of complementarymodules of a magnetic tape head 200 according to one embodiment. In thisembodiment, each module has a plurality of read/write (R/W) pairs in apiggyback configuration formed on a common substrate 204A and anoptional electrically insulative insulating layer 236. The writers 214and the readers 216 are aligned parallel to an intended direction oftravel of a tape medium thereacross to form an R/W pair, exemplified bythe R/W pair 222. Note that the intended direction of tape travel issometimes referred to herein as the direction of tape travel, and suchterms may be used interchangeably. Such direction of tape travel may beinferred from the design of the system, e.g., by examining the guides;observing the actual direction of tape travel relative to the referencepoint; etc. Moreover, in a system operable for bi-direction readingand/or writing, the direction of tape travel in both directions istypically parallel and thus both directions may be considered equivalentto each other.

Several R/W pairs 222 may be present, such as 8, 16, 32 pairs, etc. TheR/W pairs 222 as shown are linearly aligned in a direction generallyperpendicular to a direction of tape travel thereacross. However, thepairs may also be aligned diagonally, etc. Servo readers 212 arepositioned on the outside of the array of R/W pairs, the function ofwhich is well known.

Generally, the magnetic tape medium moves in either a forward or reversedirection as indicated by arrow 220. The magnetic tape medium and headassembly 200 operate in a transducing relationship in the mannerwell-known in the art. The head assembly 200 includes two thin-filmmodules 224 and 226 of generally identical construction.

Modules 224 and 226 are joined together with a space present betweenclosures 204B thereof (partially shown) to form a single physical unitto provide read-while-write capability by activating the writer of theleading module and reader of the trailing module aligned with the writerof the leading module parallel to the direction of tape travel relativethereto. When a module 224, 226 of a magnetic tape head 200 isconstructed, layers are formed in the gap 218 created above anelectrically conductive substrate 204A (partially shown), e.g., ofAlTiC, in generally the following order for the R/W pairs 222: aninsulating layer 236, a first shield 232 typically of an iron alloy suchas NiFe (-), cobalt zirconium tantalum (CZT) or Al—Fe—Si (Sendust), asensor 234 for sensing a data track on a magnetic medium, a secondshield 238 typically of a nickel-iron alloy (e.g., ˜80/20 at % NiFe,also known as permalloy), first and second writer poles 228, 230, and acoil (not shown). The sensor may be of any known type, including thosebased on magnetoresistive (MR), GMR, AMR, tunneling magnetoresistance(TMR), etc.

The first and second writer poles 228, 230 may be fabricated from highmagnetic moment materials such as ˜45/55 NiFe. Note that these materialsare provided by way of example only, and other materials may be used.Additional layers such as insulation between the shields and/or poletips and an insulation layer surrounding the sensor may be present.Illustrative materials for the insulation include alumina and otheroxides, insulative polymers, etc.

The configuration of the tape head 126 according to one embodimentincludes multiple modules, preferably three or more. In awrite-read-write (W-R-W) head, outer modules for writing flank one ormore inner modules for reading. Referring to FIG. 3, depicting a W-R-Wconfiguration, the outer modules 252, 256 each include one or morearrays of writers 260. The inner module 254 of FIG. 3 includes one ormore arrays of readers 258 in a similar configuration. Variations of amulti-module head include a R-W-R head (FIG. 4), a R-R-W head, a W-W-Rhead, etc. In yet other variations, one or more of the modules may haveread/write pairs of transducers. Moreover, more than three modules maybe present. In further approaches, two outer modules may flank two ormore inner modules, e.g., in a W-R-R-W, a R-W-W-R arrangement, etc. Forsimplicity, a W-R-W head is used primarily herein to exemplifyembodiments of the present invention. One skilled in the art apprisedwith the teachings herein will appreciate how permutations of thepresent invention would apply to configurations other than a W-R-Wconfiguration.

FIG. 5 illustrates a magnetic head 126 according to one embodiment ofthe present invention that includes first, second and third modules 302,304, 306 each having a tape bearing surface 308, 310, 312 respectively,which may be flat, contoured, etc. Note that while the term “tapebearing surface” appears to imply that the surface facing the tape 315is in physical contact with the tape bearing surface, this is notnecessarily the case. Rather, only a portion of the tape may be incontact with the tape bearing surface, constantly or intermittently,with other portions of the tape riding (or “flying”) above the tapebearing surface on a layer of air, sometimes referred to as an “airbearing”. The first module 302 will be referred to as the “leading”module as it is the first module encountered by the tape in a threemodule design for tape moving in the indicated direction. The thirdmodule 306 will be referred to as the “trailing” module. The trailingmodule follows the middle module and is the last module seen by the tapein a three module design. The leading and trailing modules 302, 306 arereferred to collectively as outer modules. Also note that the outermodules 302, 306 will alternate as leading modules, depending on thedirection of travel of the tape 315.

In one embodiment, the tape bearing surfaces 308, 310, 312 of the first,second and third modules 302, 304, 306 lie on about parallel planes(which is meant to include parallel and nearly parallel planes, e.g.,between parallel and tangential as in FIG. 6), and the tape bearingsurface 310 of the second module 304 is above the tape bearing surfaces308, 312 of the first and third modules 302, 306. As described below,this has the effect of creating the desired wrap angle α₂ of the taperelative to the tape bearing surface 310 of the second module 304.

Where the tape bearing surfaces 308, 310, 312 lie along parallel ornearly parallel yet offset planes, intuitively, the tape should peel offof the tape bearing surface 308 of the leading module 302. However, thevacuum created by the skiving edge 318 of the leading module 302 hasbeen found by experimentation to be sufficient to keep the tape adheredto the tape bearing surface 308 of the leading module 302. The trailingedge 320 of the leading module 302 (the end from which the tape leavesthe leading module 302) is the approximate reference point which definesthe wrap angle α₂ over the tape bearing surface 310 of the second module304. The tape stays in close proximity to the tape bearing surface untilclose to the trailing edge 320 of the leading module 302. Accordingly,transducers 322 may be located near the trailing edges of the outermodules 302, 306. These embodiments are particularly adapted forwrite-read-write applications.

A benefit of this and other embodiments described herein is that,because the outer modules 302, 306 are fixed at a determined offset fromthe second module 304, the inner wrap angle α₂ is fixed when the modules302, 304, 306 are coupled together or are otherwise fixed into a head.The inner wrap angle α₂ is approximately tan⁻¹(δ/W) where δ is theheight difference between the planes of the tape bearing surfaces 308,310 and W is the width between the opposing ends of the tape bearingsurfaces 308, 310. An illustrative inner wrap angle α₂ is in a range ofabout 0.3° to about 1.1°, though can be any angle required by thedesign.

Beneficially, the inner wrap angle α₂ on the side of the module 304receiving the tape (leading edge) will be larger than the inner wrapangle α₃ on the trailing edge, as the tape 315 rides above the trailingmodule 306. This difference is generally beneficial as a smaller α₃tends to oppose what has heretofore been a steeper exiting effectivewrap angle.

Note that the tape bearing surfaces 308, 312 of the outer modules 302,306 are positioned to achieve a negative wrap angle at the trailing edge320 of the leading module 302. This is generally beneficial in helpingto reduce friction due to contact with the trailing edge 320, providedthat proper consideration is given to the location of the crowbar regionthat forms in the tape where it peels off the head. This negative wrapangle also reduces flutter and scrubbing damage to the elements on theleading module 302. Further, at the trailing module 306, the tape 315flies over the tape bearing surface 312 so there is virtually no wear onthe elements when tape is moving in this direction. Particularly, thetape 315 entrains air and so will not significantly ride on the tapebearing surface 312 of the third module 306 (some contact may occur).This is permissible, because the leading module 302 is writing while thetrailing module 306 is idle.

Writing and reading functions are performed by different modules at anygiven time. In one embodiment, the second module 304 includes aplurality of data and optional servo readers 331 and no writers. Thefirst and third modules 302, 306 include a plurality of writers 322 andno data readers, with the exception that the outer modules 302, 306 mayinclude optional servo readers. The servo readers may be used toposition the head during reading and/or writing operations. The servoreader(s) on each module are typically located towards the end of thearray of readers or writers.

By having only readers or side by side writers and servo readers in thegap between the substrate and closure, the gap length can besubstantially reduced. Typical heads have piggybacked readers andwriters, where the writer is formed above each reader. A typical gap is20-35 microns. However, irregularities on the tape may tend to droopinto the gap and create gap erosion. Thus, the smaller the gap is thebetter. The smaller gap enabled herein exhibits fewer wear relatedproblems.

In some embodiments, the second module 304 has a closure, while thefirst and third modules 302, 306 do not have a closure. Where there isno closure, preferably a hard coating is added to the module. Onepreferred coating is diamond-like carbon (DLC).

In the embodiment shown in FIG. 5, the first, second, and third modules302, 304, 306 each have a closure 332, 334, 336, which extends the tapebearing surface of the associated module, thereby effectivelypositioning the read/write elements away from the edge of the tapebearing surface. The closure 332 on the second module 304 can be aceramic closure of a type typically found on tape heads. The closures334, 336 of the first and third modules 302, 306, however, may beshorter than the closure 332 of the second module 304 as measuredparallel to a direction of tape travel over the respective module. Thisenables positioning the modules closer together. One way to produceshorter closures 334, 336 is to lap the standard ceramic closures of thesecond module 304 an additional amount. Another way is to plate ordeposit thin film closures above the elements during thin filmprocessing. For example, a thin film closure of a hard material such asSendust or nickel-iron alloy (e.g., 45/55) can be formed on the module.

With reduced-thickness ceramic or thin film closures 334, 336 or noclosures on the outer modules 302, 306, the write-to-read gap spacingcan be reduced to less than about 1 mm, e.g., about 0.75 mm, or 50% lessthan commonly-used linear tape open (LTO) tape head spacing. The openspace between the modules 302, 304, 306 can still be set toapproximately 0.5 to 0.6 mm, which in some embodiments is ideal forstabilizing tape motion over the second module 304.

Depending on tape tension and stiffness, it may be desirable to anglethe tape bearing surfaces of the outer modules relative to the tapebearing surface of the second module. FIG. 6 illustrates an embodimentwhere the modules 302, 304, 306 are in a tangent or nearly tangent(angled) configuration. Particularly, the tape bearing surfaces of theouter modules 302, 306 are about parallel to the tape at the desiredwrap angle α₂ of the second module 304. In other words, the planes ofthe tape bearing surfaces 308, 312 of the outer modules 302, 306 areoriented at about the desired wrap angle α₂ of the tape 315 relative tothe second module 304. The tape will also pop off of the trailing module306 in this embodiment, thereby reducing wear on the elements in thetrailing module 306. These embodiments are particularly useful forwrite-read-write applications. Additional aspects of these embodimentsare similar to those given above.

Typically, the tape wrap angles may be set about midway between theembodiments shown in FIGS. 5 and 6.

FIG. 7 illustrates an embodiment where the modules 302, 304, 306 are inan overwrap configuration. Particularly, the tape bearing surfaces 308,312 of the outer modules 302, 306 are angled slightly more than the tape315 when set at the desired wrap angle α₂ relative to the second module304. In this embodiment, the tape does not pop off of the trailingmodule, allowing it to be used for writing or reading. Accordingly, theleading and middle modules can both perform reading and/or writingfunctions while the trailing module can read any just-written data.Thus, these embodiments are preferred for write-read-write,read-write-read, and write-write-read applications. In the latterembodiments, closures should be wider than the tape canopies forensuring read capability. The wider closures may require a widergap-to-gap separation. Therefore, a preferred embodiment has awrite-read-write configuration, which may use shortened closures thatthus allow closer gap-to-gap separation.

Additional aspects of the embodiments shown in FIGS. 6 and 7 are similarto those given above.

A 32 channel version of a multi-module tape head 126 may use cables 350having leads on the same or smaller pitch as current 16 channelpiggyback LTO modules, or alternatively the connections on the modulemay be organ-keyboarded for a 50% reduction in cable span. Over-under,writing pair unshielded cables may be used for the writers, which mayhave integrated servo readers.

The outer wrap angles α₁ may be set in the drive, such as by guides ofany type known in the art, such as adjustable rollers, slides, etc. oralternatively by outriggers, which are integral to the head. Forexample, rollers having an offset axis may be used to set the wrapangles. The offset axis creates an orbital arc of rotation, allowingprecise alignment of the wrap angle α₁.

To assemble any of the embodiments described above, conventional u-beamassembly can be used. Accordingly, the mass of the resultant head may bemaintained or even reduced relative to heads of previous generations. Inother approaches, the modules may be constructed as a unitary body.Those skilled in the art, armed with the present teachings, willappreciate that other known methods of manufacturing such heads may beadapted for use in constructing such heads. Moreover, unless otherwisespecified, processes and materials of types known in the art may beadapted for use in various embodiments in conformance with the teachingsherein, as would become apparent to one skilled in the art upon readingthe present disclosure.

As a tape is run over a module, it is preferred that the tape passessufficiently close to magnetic transducers on the module such thatreading and/or writing is efficiently performed, e.g., with a low errorrate. According to some approaches, tape tenting may be used to ensurethe tape passes sufficiently close to the portion of the module havingthe magnetic transducers. To better understand this process, FIGS. 8A-8Cillustrate the principles of tape tenting. FIG. 8A shows a module 800having an upper tape bearing surface 802 extending between oppositeedges 804, 806. A stationary tape 808 is shown wrapping around the edges804, 806. As shown, the bending stiffness of the tape 808 lifts the tapeoff of the tape bearing surface 802. Tape tension tends to flatten thetape profile, as shown in FIG. 8A. Where tape tension is minimal, thecurvature of the tape is more parabolic than shown.

FIG. 8B depicts the tape 808 in motion. The leading edge, i.e., thefirst edge the tape encounters when moving, may serve to skive air fromthe tape, thereby creating a subambient air pressure between the tape808 and the tape bearing surface 802. In FIG. 8B, the leading edge isthe left edge and the right edge is the trailing edge when the tape ismoving left to right. As a result, atmospheric pressure above the tapeurges the tape toward the tape bearing surface 802, thereby creatingtape tenting proximate each of the edges. The tape bending stiffnessresists the effect of the atmospheric pressure, thereby causing the tapetenting proximate both the leading and trailing edges. Modeling predictsthat the two tents are very similar in shape.

FIG. 8C depicts how the subambient pressure urges the tape 808 towardthe tape bearing surface 802 even when a trailing guide 810 ispositioned above the plane of the tape bearing surface.

It follows that tape tenting may be used to direct the path of a tape asit passes over a module. As previously mentioned, tape tenting may beused to ensure the tape passes sufficiently close to the portion of themodule having the magnetic transducers, preferably such that readingand/or writing is efficiently performed, e.g., with a low error rate.

Magnetic tapes may be stored in tape cartridges that are, in turn,stored at storage slots or the like inside a data storage library. Thetape cartridges may be stored in the library such that they areaccessible for physical retrieval. In addition to magnetic tapes andtape cartridges, data storage libraries may include data storage drivesthat store data to, and/or retrieve data from, the magnetic tapes.Moreover, tape libraries and the components included therein mayimplement a file system which enables access to tape and data stored onthe tape.

File systems may be used to control how data is stored in, and retrievedfrom, memory. Thus, a file system may include the processes and datastructures that an operating system uses to keep track of files inmemory, e.g., the way the files are organized in memory. Linear TapeFile System (LTFS) is an exemplary format of a file system that may beimplemented in a given library in order to enables access to complianttapes. It should be appreciated that various embodiments herein can beimplemented with a wide range of file system formats, including forexample IBM Spectrum Archive Library Edition (LTFS LE). However, toprovide a context, and solely to assist the reader, some of theembodiments below may be described with reference to LTFS which is atype of file system format. This has been done by way of example only,and should not be deemed limiting on the invention defined in theclaims.

A tape cartridge may be “loaded” by inserting the cartridge into thetape drive, and the tape cartridge may be “unloaded” by removing thetape cartridge from the tape drive. Once loaded in a tape drive, thetape in the cartridge may be “threaded” through the drive by physicallypulling the tape (the magnetic recording portion) from the tapecartridge, and passing it above a magnetic head of a tape drive.Furthermore, the tape may be attached on a take-up reel (e.g., see 121of FIG. 1A above) to move the tape over the magnetic head.

Once threaded in the tape drive, the tape in the cartridge may be“mounted” by reading metadata on a tape and bringing the tape into astate where the LTFS is able to use the tape as a constituent componentof a file system. Moreover, in order to “unmount” a tape, metadata ispreferably first written on the tape (e.g., as an index), after whichthe tape may be removed from the state where the LTFS is allowed to usethe tape as a constituent component of a file system. Finally, to“unthread” the tape, the tape is unattached from the take-up reel and isphysically placed back into the inside of a tape cartridge again. Thecartridge may remain loaded in the tape drive even after the tape hasbeen unthreaded, e.g., waiting for another read and/or write request.However, in other instances, the tape cartridge may be unloaded from thetape drive upon the tape being unthreaded, e.g., as described above.

Magnetic tape is a sequential access medium. Thus, new data is writtento the tape by appending the data at the end of previously written data.It follows that when data is recorded in a tape having only onepartition, metadata (e.g., allocation information) is continuouslyappended to an end of the previously written data as it frequentlyupdates and is accordingly rewritten to tape. As a result, the rearmostinformation is read when a tape is first mounted in order to access themost recent copy of the metadata corresponding to the tape. However,this introduces a considerable amount of delay in the process ofmounting a given tape.

To overcome this delay caused by single partition tape mediums, the LTFSformat includes a tape that is divided into two partitions, whichinclude an index partition and a data partition. The index partition maybe configured to record metadata (meta information), e.g., such as fileallocation information (Index), while the data partition may beconfigured to record the body of the data, e.g., the data itself.

Looking to FIG. 9, a magnetic tape 900 having an index partition 902 anda data partition 904 is illustrated according to one embodiment. Asshown, data files and indexes are stored on the tape. The LTFS formatallows for index information to be recorded in the index partition 902at the beginning of tape 906, as would be appreciated by one skilled inthe art upon reading the present description.

As index information is updated, it preferably overwrites the previousversion of the index information, thereby allowing the currently updatedindex information to be accessible at the beginning of tape in the indexpartition. According to the specific example illustrated in FIG. 9, amost recent version of metadata Index 3 is recorded in the indexpartition 902 at the beginning of the tape 906. Conversely, all threeversion of metadata Index 1, Index 2, Index 3 as well as data File A,File B, File C, File D are recorded in the data partition 904 of thetape. Although Index 1 and Index 2 are old (e.g., outdated) indexes,because information is written to tape by appending it to the end of thepreviously written data as described above, these old indexes Index 1,Index 2 remain stored on the tape 900 in the data partition 904 withoutbeing overwritten.

The metadata may be updated in the index partition 902 and/or the datapartition 904 the same or differently depending on the desiredembodiment. According to some embodiments, the metadata of the indexand/or data partitions 902, 904 may be updated in response to the tapebeing unmounted, e.g., such that the index may be read quickly from theindex partition when that tape is mounted again. The metadata ispreferably also written in the data partition 904 so the tape may bemounted using the metadata recorded in the data partition 904, e.g., asa backup option.

According to one example, which is no way intended to limit theinvention, LTFS LE may be used to provide the functionality of writingan index in the data partition when a user explicitly instructs thesystem to do so, or at a time designated by a predetermined period whichmay be set by the user, e.g., such that data loss in the event of suddenpower stoppage can be mitigated.

As track density increases, resolution of dimensional stability issuesbecomes more important, and tolerance control is reaching the limit ofits effectiveness. Particularly, there is a limit to the precision atwhich components can be fabricated. As dimensions of components becomesmaller and smaller, that limit will be reached.

As mentioned above, in tape storage, the magnetic head dimensions arenot only different from drive to drive, but each head may also changeover time due to factors such as thermal expansion, relaxation ofstresses within the head, etc. Moreover, tape lateral contraction andexpansion is a well-known phenomenon that occurs due to a plethora ofeffects, including absorption of water, thermal expansion andcontraction, etc.

More permanent changes in media lateral dimensions may also occur, suchas long-term media creep (also known in the art as “aging”), which tendsto occur over time when a tape is wound around a hub of a tapecartridge. Long-term media creep is particularly problematic whendealing with tape dimensional stability issues, as the two ends of thetape exhibit creep in different ways. The inner wraps of tape positionedclosest to the cartridge hub tend to expand laterally over time due tothe compressive stresses exerted thereon by the wraps of tape woundaround them. Wraps positioned toward the outer diameter of the spool oftape are under less compressive stress, but are under higher tensilestresses, which tends to cause lateral contraction of the tape, i.e.,the tape becomes narrower over time. Accordingly, the ends of the tapeexhibit oppositely-oriented lateral dimensional changes.

When the dimensions of the tape, the head, or both change, variousissues arise. During writing, the likelihood of overwriting shingledtracks increases. Overwritten data is often unrecoverable. Likewise,during readback, if readers are no longer over the tracks to be read,reading errors increase.

FIG. 10 depicts the effect of a change in dimension of a head and/ortape after writing has occurred. For simplicity, five data tracks areshown, labeled Data Track 1 through Data Track 5. As shown, the datatracks are written at a certain spacing, referred to as the writingcondition. However, sometime after writing, the tape has expanded due tofactors such as temperature, humidity, creep, etc. Assume the readershave the same spacing (pitch) as the writers that wrote the data tracks.The track following system centers the middle reader on the middletrack, but the outer readers are then partially off-track due to theexpanded condition of the tape. Accordingly, not only are the outerreaders less influenced by the magnetic transitions of the outer datatracks, but shingled tracks adjacent the intended tracks influence thereaders, creating noise. Thus, the misregistration results in a higheramount of read errors for tracks positioned toward the ends of thearray. FIG. 11 is a chart exemplifying the increase in error ratestoward outer ends of a reader array due to a misregistration of thereaders toward the outer ends of the array. There is no position thatthe head may move to that will improve readback.

Similarly, if the magnetic head expands or contracts, or if due to headmanufacturing tolerances the writer head and reader head have differentspacing, similar misregistration can occur, even if the tape has notchanged. Where both the tape and reading head have changed in oppositedimensions, e.g., one is contracted while the other is expanded, themisregistration problem is compounded. Thus, in either case the readbackof that data is impaired.

In order to overcome the limitations mentioned above, new techniques tomanage the stability of head and/or media are needed. Various techniquesand approaches for managing head and media dimensional stability arepresented herein.

Referring again to FIGS. 2A-4, typical tape drives have multiplemodules, and at least two servo readers on each module. These servoreaders and track following module of the controller decode respectiveservo patterns and assist in positioning the data transducers at theappropriate locations for reading and/or writing. In an ideal situation,the track following module would indicate that both servo readers arereading the same position on their respective servo tracks. However,this is rarely the case, for the many reasons enumerated above.

Fortunately, changes in dimensions of the head, the media, or both canbe detected by comparing the difference in the servo readermeasurements. This measurement from the servo readers is one method thatcan be used to determine variations in heads and media, and will bereferred to herein as Servo Band Difference (SBD). SBD information mayinclude the SBD measurement itself and/or information derived from theSBD measurements.

Various approaches for managing dimensional stability issues arepresented below. In general, the approaches include three individuallynovel components that together may form one overall general solution.The first component includes characterizing a magnetic recording tape(“tape”) of a cartridge, e.g., at initialization of the cartridge,sometime thereafter, and/or as a recharacterization. The secondcomponent includes utilizing this information during writing, and mayalso include recording SBD information during writing e.g., to a dataset information table (DSIT). The third component includes utilizing theSBD information and/or DSIT information during reading.

To measure SBD, servo readers on the same module read respective servopatterns on the media. In the ideal case, both servo readers wouldmeasure the same position on their relative servo pattern. However,media and heads are rarely ideal, and therefore any deviation from thisideal case can be determined by comparing the position measurements fromthe two servo channels. If SBD gets larger, this implies that the tapehas contracted in the lateral direction and/or that the head hasexpanded. Likewise, if SBD gets smaller, this implies that the tape hasexpanded in the lateral direction and/or that the head has contracted.

The SBD measurements may be used to characterize a magnetic recordingtape. Referring to FIG. 12, a flowchart of a method 1200 forcharacterizing a magnetic recording tape of a tape cartridge is shown.The method 1200 may be performed in accordance with the presentinvention in any of the environments depicted in FIGS. 1-9, amongothers, in various approaches. Of course, more or less operations thanthose specifically described in FIG. 12 may be included in method 1200,as would be understood by one of skill in the art upon reading thepresent descriptions.

Each of the steps of the method 1200 may be performed by any suitablecomponent of the operating environment. For example, in variousembodiments, the method 1200 may be partially or entirely performed by atape drive, or some other device having one or more processors therein.The processor, e.g., processing circuit(s), chip(s), and/or module(s)implemented in hardware and/or software, and preferably having at leastone hardware component may be utilized in any device to perform one ormore steps of the method 1200. Illustrative processors include, but arenot limited to, a central processing unit (CPU), an application specificintegrated circuit (ASIC), a field programmable gate array (FPGA), etc.,combinations thereof, or any other suitable computing device known inthe art.

This process may be performed when a new tape is being prepared forfirst use. For example, this process may be added to a conventionalcartridge initialization process. This process may also be performedwhen a data band or data bands on a used tape is ready for overwriting.

As shown in FIG. 12, method 1200 may initiate with operation 1202, whereSBD measurements are made at various locations along a length of amagnetic recording tape using a magnetic head having servo readers ofknown pitch. Because the spacing is not generally constant along thelength of the tape, the observed SBD measurements are typicallydifferent as the tape move from BOT toward EOT. Without wishing to bebound by any theory, it is believed that this change is due at least inpart to the pack stresses that are imputed in tape when stored in thecartridge. SBD measurements may be taken for some, and preferably foreach of the data bands on the tape. Preferably, the SBD measurements aretaken at various locations along about an entire length of the magneticrecording tape, but, in some approaches, only a portion of the length ofthe tape is characterized. The tape is ideally maintained at aboutconstant tension while measuring the SBD to minimize tension-induceddimensional changes of the tape. The constant tension is preferablysimilar to the preferred tension for read and/or write operations on themagnetic tape.

In one approach, while holding the tape tension about fixed, the tapedrive moves the tape from BOT to EOT while making measurements of SBD.Since the SBD tends to change from BOT to EOT, multiple measurements arepreferably made. In general, any granularity of measurement interval canbe applied, with higher numbers of SBD measurements providing moreinformation for later use. Preferably, at least 100 SBD measurements aretaken between BOT and EOT for each data band, and more preferably atleast 200 SBD measurements are taken between BOT and EOT for each databand, though less than 100 measurements may be taken in some approaches.

Note that servo reader pitch varies from head to head, and therefore,the raw SBD measurements do not typically reflect the actual servo trackspacing. Said another way, wider or narrower servo pitch on the headthan the assumed pitch would cause an error in the measurement of thecurrent media spacing value. Accordingly, during this process, the pitchof the servo readers on the head is preferably known, and used to adjust(compensate) the SBD values so that the SBD values more accuratelyreflect the actual media spacing characteristics. The pitch of the servoreaders corresponds directly to the spacing of the servo readersrelative to each other, and may be center-to-center pitch, edge-to-edgepitch, etc.

The pitch of the servo readers may be derived or obtained in anysuitable known manner. Typically, this value is stored in the memory ofeach drive during manufacture thereof. In one approach, the pitch ismeasured for each drive at manufacturing and placed in a non-volatilearea of drive memory such as with the vital product data (VPD). Thishead calibration can be performed in multiple ways, such as measurementwith an atomic force microscope (AFM) using stages, the use of areference tape having servo tracks of known spacing, or any other methodwhich provides a measurement of transducers relative to othertransducers. In another approach, the pitch is measured for a driveafter the drive has been built, and optionally in use. In a preferredapproach, a reference tape may be used.

By using the pitch value stored in the VPD, when a tape is characterizedusing process 1200, the measurements observed can be compensatedaccording to the head spacing value stored in VPD, thus ensuring thatthe measurements taken, and corresponding values ultimately written tothe cartridge memory (CM), are representative of the cartridge, and notunduly influenced by the head making the measurement.

Additionally, by using temperature and/or humidity sensors in the drive(or external sensors with information communicated to the drive), theeffects of the local temperature and/or humidity can also be compensatedfor. For example, if the humidity is high, then tape expands and thecartridge is initialized at this high humidity condition. It is desiredthat the stored SBD values represent a nominal condition in headspacing, temperature, and humidity.

In operation 1204, the SBD measurements and/or derivatives thereof(collectively referred to herein as “SBD information”) are stored inassociation with the tape cartridge. Preferably, the SBD informationincludes a position along the tape where each SBD measurement was taken,in association with the corresponding SBD measurement. For example,Linear Tape Open (LTO) linear positioning (LPOS) information may bestored in association with each SBD measurement. Accordingly, arepresentation of the media spacing characteristics at the time ofperforming method 1200 is stored for later use.

Any of a plurality of storage techniques may be used to store SBDinformation, such as storage of raw points; fitting of the measurementsto a function (linear, polynomial, spline, etc.) and then storing thecoefficients or describing variables; etc. The SBD information can bestored in any suitable location where it can be referenced at a latertime. The SBD information is preferably written to the CM of thecartridge. Other locations for storage of the SBD information include onthe tape itself, e.g., in the header information; on a removable storagedevice of the cartridge, e.g., an SD card; in a database of informationabout tape cartridges, e.g., in a library database; in cloud-basedstorage; etc.

The method 1200 may be performed as part of a cartridge initializationprocedure. For example, in addition to performing conventional specialoperations during the first load of a brand-new cartridge, theoperations of method 1200 may be performed during the cartridgeinitialization process.

The method 1200 may also be invoked at times other than the first load.For example, the timing for performing method 1200 may correspond toother operations, such as changing the format of the tape, after agarbage collection process renders all data on the tape deleted, etc.Characterizing or recharacterizing the tape at times other than thefirst load may be useful to reset the SBD information to account for anycreep that has occurred in the media since the previous initialization.Other operations, such as those that are completely destructive, such asthe format command, may be considered as appropriate times to reissuethe cartridge initialization.

FIG. 13 is a flowchart of a method 1300 for characterizing a magneticrecording tape of a tape cartridge in one illustrative approach. Themethod 1300 may be performed in accordance with the present invention inany of the environments depicted in FIGS. 1-12, among others, in variousapproaches. Of course, more or less operations than those specificallydescribed in FIG. 13 may be included in method 1300, as would beunderstood by one of skill in the art upon reading the presentdescriptions.

Each of the steps of the method 1300 may be performed by any suitablecomponent of the operating environment. For example, in variousembodiments, the method 1300 may be partially or entirely performed by atape drive, or some other device having one or more processors therein.The processor, e.g., processing circuit(s), chip(s), and/or module(s)implemented in hardware and/or software, and preferably having at leastone hardware component may be utilized in any device to perform one ormore steps of the method 1300. Illustrative processors include, but arenot limited to, a central processing unit (CPU), an application specificintegrated circuit (ASIC), a field programmable gate array (FPGA), etc.,combinations thereof, or any other suitable computing device known inthe art.

As shown in FIG. 13, method 1300 may initiate with operation 1302, wherea cartridge is loaded into a tape drive. At decision 1304, adetermination is made as to whether the tape has been initialized, e.g.,using the method 1200 of FIG. 12, and SBD information is available forthe tape. If so, the cartridge is deemed ready for read and/or writeoperations. See operation 1306. If SBD information is not available, themethod 1300 proceeds with operation 1308 where the SBD is measured atmultiple locations of the tape using about constant tension. Inoperation 1310, the measurements are compensated for any of a variety ofparameters. For example, the measurements may be compensated due tomeasurement bias from the head dimensions, and namely the servo readerpitch. The compensation may also and/or alternatively have a temperatureand/or humidity component. In operation 1312, the SBD information isstored, preferably in the CM of the cartridge, but may be in otherlocations such as on the media itself, in a removable memory coupled tothe cartridge such as an SD card, etc. The cartridge is deemed ready forread and/or write operations. See operation 1314.

FIG. 14 is a chart 1400 that illustrates exemplary sample SBD referencevalues from BOT to EOT gathered during performance of the method 1200 ofFIG. 12. As shown, the SBD measurements are highest at BOT and turnslightly negative by EOT. As noted above, where SBD is larger, thisimplies that the tape has contracted in the lateral direction in thetime since the servo tracks were written. Likewise, where SBD getssmaller, this implies that the tape has expanded in the lateraldirection.

The SBD information stored in association with the tape and itscartridge can then be used for other things, such as reading andwriting.

During writing, the stored SBD information may be retrieved, e.g., fromthe CM, and loaded into the drive memory for use as reference values forthe desired SBD for the current writing operation. Because most tapeformats utilize shingling, the current tracks partially overwritepreviously written tracks. The amount of shingling must be preciselycontrolled, or else too much of the previous track will be overwritten,and the data written to those previous tracks will become unreadable andthe data irretrievably lost. As tape and head dimensional changes affectthe location of written tracks, it is critical that the current writingoperation does not excessively trim, or narrow, the previously writtendata.

FIG. 15 is a flowchart of a method 1500 for controlling writing to amagnetic recording tape of a tape cartridge. The method 1500 may beperformed in accordance with the present invention in any of theenvironments depicted in FIGS. 1-14, among others, in variousapproaches. Of course, more or less operations than those specificallydescribed in FIG. 15 may be included in method 1500, as would beunderstood by one of skill in the art upon reading the presentdescriptions.

Each of the steps of the method 1500 may be performed by any suitablecomponent of the operating environment. For example, in variousembodiments, the method 1500 may be partially or entirely performed by atape drive, or some other device having one or more processors therein.The processor, e.g., processing circuit(s), chip(s), and/or module(s)implemented in hardware and/or software, and preferably having at leastone hardware component may be utilized in any device to perform one ormore steps of the method 1500. Illustrative processors include, but arenot limited to, a central processing unit (CPU), an application specificintegrated circuit (ASIC), a field programmable gate array (FPGA), etc.,combinations thereof, or any other suitable computing device known inthe art.

The method 1500 may be performed in response to receiving a request towrite to a tape of a tape cartridge. Conventional operations aretypically performed in addition to the steps below, including loadingthe tape in the tape drive, mounting the tape, spooling the tape to theproper location for writing, processing index information about the dataon the tape, etc.

Operation 1502 includes retrieving servo band difference informationabout the tape cartridge. Again, the servo band difference informationmay be retrieved from any source, such as CM of the cartridge, from thetape itself, from a remote database, etc.

Operation 1504 includes measuring, using servo readers of the magnetichead of the tape drive, a servo band difference at various locationsalong a length of a magnetic recording tape of the tape cartridge. Forexample, the servo band difference measurements may be taken as the tapeis indexed to the writing position. In another approach, measurementsmay be taken at points along the entire tape or selected portionthereof, prior to writing. Preferably, at least some of the measuring isperformed during the writing operation.

Operation 1506 includes comparing the servo band difference measurementsand/or derivatives thereof to the retrieved SBD information (such asvalues in and/or derived from the SBD information). For example, acurrent measurement may be compared to the SBD value recorded in the CM.

Operation 1508 includes controlling a writing operation based at leastin part on a result of the comparing. Any parameter associated with thewriting operation may be controlled in operation 1508, such assuspending and/or canceling writing in response to a result of thecomparing if operation 1506 being indicative of potential off-trackwriting; adjusting an operating condition of the tape drive for reducingoccurrence of off-track writing such as by changing a width of the tapeby adjusting tape tension and/or heating or cooling the tape, adjustingpitches between transducers of the magnetic head e.g., by inducingthermal expansion of the head using an integrated heating device,inducing expansion or contraction of the head using a piezo device,etc.; tilting the axis of the array of transducers away fromperpendicular to the direction of tape travel; etc.

Where the comparison of operation 1508 is performed prior to writing,the writing operation may be suspended, e.g., not started, in responseto a result of the comparing being indicative of potential off-trackwriting, and writing conditions may be adjusted in an attempt to improvethe results of the comparison. If the result of the comparing is in apredefined range indicating that off-track writing is sure to occur, theentire writing operation may be canceled. If the result of the comparingindicates that only a portion of the tape is unsuitable for writing,writing may be performed in areas of the tape away from that portion.

Where the comparison is performed during writing, the writing operationmay be suspended in response to a result of the comparing beingindicative of potential (including actual) off-track writing.

In one approach, if the difference between the current SBD informationand the stored SBD information is less than an amount which can betolerated by the format being used, then writing is allowed to continue.If the difference is greater than this amount, then the writing isstopped to prevent overwriting of adjacent tracks. This is a similarsituation as when writing is stopped for excessive Position Error Signal(PES) to prevent overwriting of adjacent tracks. If the SBD exceeds athreshold set for the particular format being used, then multipleoptions are available. One option is to simply stop writing, and if thestop writing distance is long enough, the write operation may cease witha permanent error. In various approaches, this error does cause thewriting to stop, but it protects previously written data that would haveotherwise been overwritten if not for the stop writing condition.

As noted above, another option to control the writing operation is toutilize a scheme to adjust the SBD, such as varying the tape tension.Note that any other technique to adjust the SBD can also and/oralternatively be utilized. In one approach, the tension is continuouslyadjusted regardless of SBD relative to the reference CM values. Inanother approach, the tension is only adjusted if the SBD exceeds athreshold from the reference values. A potential advantage of waiting toadjust tension until after SBD exceeds a threshold is that utilizingtension has some negative side effects, such as creating a tape packthat has a higher stress value. By delaying the use of tension until itis absolutely needed, the negative side effects can be deferred untilnecessary.

The SBD measurements created in operation 1504 and/or derivativesthereof are preferably stored on the tape cartridge, e.g., in a DSIT, inCM, and/or on tape.

In a preferred approach, for the measurements taken during writing(which may include and/or be measurements taken prior or subsequent towriting but after the tape is loaded and before it is subsequentlyunloaded, the actual measurements of SBD and/or derivatives thereof atthe time of writing are recorded in the DSIT along with details aboutthe data written on tape. This current SBD information is preferablystored regardless of the value written in the CM. The current SBDinformation recorded in the DSIT may be different from the SBDinformation written to the CM during cartridge initialization, as therewill typically be some difference in SBD between the initial cartridgecharacterization and the actual writing of use data. It is important tonote that the SBD measured during writing should not be modified by anyknown head parameters from VPD and/or temperature/humidity effects.While the earlier-stored head spacing values were useful duringcartridge calibration to get accurate measurements of the cartridge intothe CM, these values should not be recorded during writing, as themeasurement of SBD during writing is a statement of the head and mediacondition at the time of writing. For example, if a “wide” head having awider than ideal servo spacing were used during cartridgeinitialization, it would be desirable to remove the bias that the widehead has for the measurement. However, during the actual writingprocess, this wide head writes tracks in locations that are fartherapart and the drive should take the appropriate action for thisbehavior. The CM holds the reference locations that an ideal writerwould observe while writing, while the DSIT contains information aboutthe actual conditions observed during writing.

FIG. 16 is a flowchart of a method 1600 for one exemplary mode of use.The method 1600 may be performed in accordance with the presentinvention in any of the environments depicted in FIGS. 1-15, amongothers, in various approaches. Of course, more or less operations thanthose specifically described in FIG. 16 may be included in method 1600,as would be understood by one of skill in the art upon reading thepresent descriptions.

Each of the steps of the method 1600 may be performed by any suitablecomponent of the operating environment. For example, in variousembodiments, the method 1600 may be partially or entirely performed by atape drive, or some other device having one or more processors therein.The processor, e.g., processing circuit(s), chip(s), and/or module(s)implemented in hardware and/or software, and preferably having at leastone hardware component may be utilized in any device to perform one ormore steps of the method 1600. Illustrative processors include, but arenot limited to, a central processing unit (CPU), an application specificintegrated circuit (ASIC), a field programmable gate array (FPGA), etc.,combinations thereof, or any other suitable computing device known inthe art.

In operation 1602, before and/or during writing, the current SBD ismeasured by the writing head. Decision 1604 indicates whether thetension can be varied. If the tension can be varied, the tension ismodulated to drive the current SBD toward the reference SBD stored inthe CM for this particular location on tape. See operation 1606. If thetension cannot be varied, the SBD is simply observed. See operation1608. At decision 1610, a determination is made as to whether thecurrent SBD is different from the reference SBD stored in the CM by morethan a specific amount, e.g., a predefined amount that is indicative ofpotential off-track writing. If the current SBD is not different fromthe reference SBD by at least the specific amount, writing continues inoperation 1612. The current SBD may also be stored in the DSIT. If thecurrent SBD is different from the reference SBD by at least the specificamount, writing is discontinued in operation 1614 to prevent off-trackwriting and/or track trimming.

The present description will now turn to using SBD information duringreading operations. During reading, the SBD information stored duringwriting, such as the values of SBD observed during writing, are read,e.g., from DSIT information. In order to achieve the best readingpossible, it is desirable that all of the readers are centered abovetheir respective tracks on tape. This is best achieved when the SBDobserved during reading matches the value that was stored, e.g., in theDSIT during writing. When all of the readers are centered above theirrespective tracks on tape, error rates are lower, and less errorcorrection processing, such as Error Correction Code (ECC) processing isneeded. Therefore, the error processing capabilities can dedicate moreresources to other operations such as dealing with electronic noise,media defects, and the like. The reading operation may be controlled forsuch things as suspending writing, modifying the SBD during reading inan attempt to match the SBD from the SBD information stored when thedata was written, etc.

FIG. 17 is a flowchart of a method 1700 for controlling writing to amagnetic recording tape of a tape cartridge. The method 1700 may beperformed in accordance with the present invention in any of theenvironments depicted in FIGS. 1-16, among others, in variousapproaches. Of course, more or less operations than those specificallydescribed in FIG. 17 may be included in method 1700, as would beunderstood by one of skill in the art upon reading the presentdescriptions.

Each of the steps of the method 1700 may be performed by any suitablecomponent of the operating environment. For example, in variousembodiments, the method 1700 may be partially or entirely performed by atape drive, or some other device having one or more processors therein.The processor, e.g., processing circuit(s), chip(s), and/or module(s)implemented in hardware and/or software, and preferably having at leastone hardware component may be utilized in any device to perform one ormore steps of the method 1700. Illustrative processors include, but arenot limited to, a central processing unit (CPU), an application specificintegrated circuit (ASIC), a field programmable gate array (FPGA), etc.,combinations thereof, or any other suitable computing device known inthe art.

The method 1700 may be performed in response to receiving a request toread from a tape of a tape cartridge. Conventional operations aretypically performed in addition to the steps below, including loadingthe tape in the tape drive, mounting the tape, processing indexinformation about the data on the tape, spooling the tape to the properlocation for reading, etc.

Operation 1702 includes retrieving servo band difference informationabout the tape cartridge. Again, the SBD information may be retrievedfrom any source, such as CM of the cartridge, from the tape itself, froma remote database, etc. Here, the retrieved SBD information ispreferably indicative of a condition of the magnetic recording tapecontemporaneously with the writing of the data to be read, i.e., asmeasured at some period between loading for the writing operation andsubsequently unloading the tape cartridge. For example, SBD informationgathered before (or equivalently, after) writing the data may beretrieved, so that the writing operation can be controlled if needed.

Operation 1704 includes measuring, using servo readers of the magnetichead of the tape drive that will perform the reading operation, a servoband difference at various locations along a length of a magneticrecording tape of the tape cartridge. For example, the servo banddifference measurements may be taken as the tape is indexed to thereading position. In another approach, measurements may be taken atpoints along the entire tape or selected portion thereof, prior toreading. Preferably, at least some of the measuring is performed duringthe reading operation.

Operation 1706 includes comparing the SBD measurements and/orderivatives thereof to the retrieved SBD information (such as values inand/or derived from the retrieved SBD information). For example, thecurrent measurement may be compared to the SBD information recorded inthe DSIT when the data to be read was written to the tape.

Operation 1708 includes controlling a reading operation based at leastin part on a result of the comparing. Any parameter associated with thereading operation may be controlled in operation 1708, such assuspending and/or canceling reading in response to a result of thecomparing if operation 1706 being indicative of potential off-trackreading; performing error recovery in response to a result of thecomparing if operation 1706 being indicative of potential off-trackreading; adjusting an operating condition of the tape drive for reducingoccurrence of off-track reading such as by changing a width of the tapeby adjusting tape tension and/or heating or cooling the tape, adjustingpitches between transducers of the magnetic head e.g., by inducingthermal expansion of the head using an integrated heating device, etc.;etc.

As noted above, tension or other technique may be used to modify the SBDduring reading to match the SBD written in the DSIT. However, if it isdesired not to change the stress of the packed tape, then it may bedesirable to not modify the tension, which could result in permanentread errors. If a large difference is observed between the current SBDand the SBD recorded in the DSIT, temporary error recovery may beperformed, such as increasing the tension range (or utilization oftension modulation, even if the initial setting was to leave tensionfixed). Typically, an error recovery operation is preferable to an errorcondition. However, other drives that have different reader head spacingvalues (pitch), or other temperature and/or humidity conditions, mayalso change the apparent spacing during read which may more closelyalign the readers with their respective tracks.

FIG. 18 is a flowchart of a method 1800 for one exemplary mode of use.The method 1800 may be performed in accordance with the presentinvention in any of the environments depicted in FIGS. 1-17, amongothers, in various approaches. Of course, more or less operations thanthose specifically described in FIG. 18 may be included in method 1800,as would be understood by one of skill in the art upon reading thepresent descriptions.

Each of the steps of the method 1800 may be performed by any suitablecomponent of the operating environment. For example, in variousembodiments, the method 1800 may be partially or entirely performed by atape drive, or some other device having one or more processors therein.The processor, e.g., processing circuit(s), chip(s), and/or module(s)implemented in hardware and/or software, and preferably having at leastone hardware component may be utilized in any device to perform one ormore steps of the method 1800. Illustrative processors include, but arenot limited to, a central processing unit (CPU), an application specificintegrated circuit (ASIC), a field programmable gate array (FPGA), etc.,combinations thereof, or any other suitable computing device known inthe art.

In operation 1802, before and/or during reading, the current SBD ismeasured by the reading head. Decision 1804 determines whether thetension can be varied. If the tension can be varied, the tension ismodulated to drive the current SBD toward the reference SBD stored inthe DSIT for this particular location on tape. See operation 1806. Ifthe tension cannot be varied, the SBD is simply observed. See operation1808. At decision 1810, a determination is made as to whether thecurrent SBD is different from the reference SBD stored in the DSIT bymore than a specific amount, e.g., a predefined amount that isindicative of potential off-track reading. If the current SBD is notdifferent from the reference SBD by at least the specific amount,reading continues in operation 1812. If the current SBD is differentfrom the reference SBD by at least the specific amount, remedialmeasures are taken in operation 1814, such as performing error recovery,such as increasing a tension range, etc. A customer warning may also, oralternatively, be output.

In addition to generation and use of SBD data as described in detailabove, one further advantage is that SBD information, e.g., stored inthe CM during cartridge initialization, may be used as reference valuesto measure the amount of creep that a tape cartridge has experienced.For example, assume that data is stored on a tape of a tape cartridge,and then a user wanted to know five years later what the condition ofthe tape is in terms of the extent of creep, the tape may be loaded intoa drive and the SBD measured from BOT to EOT or some lengththerebetween, e.g., in a motion similar to the initial cartridgeinitialization. Even if this verification is performed on a differentdrive than the drive used for initialization of this particular tapecartridge, because both drives have their respective head spacingvalues, the current measurements can be compensated. Additionally,temperature and/or humidity compensations may also be applied to moreclosely approximate the tape spacing condition in the nominal situation.What remains is a comparison of the tape condition at initialization tothe present condition without the influence of the initializing head orthe verifying head, and/or other environmental variations. It may bedesired to quantify the amount of creep, the amount of environmentaldifference, or a combination of both, that has occurred since tapeinitialization. By recording initial SBD measurements, along withinitial head spans, temperature, and humidity, these differences can beobtained. This ability to quantify creep or other changes is a usefulfeature for large tape library installations where it is desired toperiodically measure tapes to ensure that their creep rates are asexpected. Determinations may also be made to detect whether specifictapes have crept more than a desired amount, and these tapes can bemarked for copying to alternate tapes before excessive creep rendersthese tapes unreadable. After copying the data to alternate tapes, thesehigh creep tapes may be reinitialized/reformatted, which resets thereference values in the CM and renders any aging/creep that happenedpreviously as irrelevant.

FIG. 19 is a flowchart of a method 1900 for characterizing the presentstate of a tape of a tape cartridge relative to an earlier statethereof. This process 1900 enables determination of the effects of agingof the tape, the present effect of environmental conditions on the tape,etc. The method 1900 may be performed in accordance with the presentinvention in any of the environments depicted in FIGS. 1-18, amongothers, in various approaches. Of course, more or less operations thanthose specifically described in FIG. 19 may be included in method 1900,as would be understood by one of skill in the art upon reading thepresent descriptions.

Each of the steps of the method 1900 may be performed by any suitablecomponent of the operating environment. For example, in variousembodiments, the method 1900 may be partially or entirely performed by atape drive, or some other device having one or more processors therein.The processor, e.g., processing circuit(s), chip(s), and/or module(s)implemented in hardware and/or software, and preferably having at leastone hardware component may be utilized in any device to perform one ormore steps of the method 1900. Illustrative processors include, but arenot limited to, a central processing unit (CPU), an application specificintegrated circuit (ASIC), a field programmable gate array (FPGA), etc.,combinations thereof, or any other suitable computing device known inthe art.

The method 1900 may be performed in response to receiving a request toread from and/or write to a tape of a tape cartridge. The request mayinclude a health check. The method 1900 allows the drive, library and/oruser to determine whether the read and/or operation will be successful,or may result in excessive overwriting and/or read errors duemisregistration between the transducers and data tracks. In anotherapproach, the method 1900 may be performed in response to receiving arequest to simply perform a health check. In yet another approach,several of the operations of the method 1900 may be performedautomatically without receiving a request for a health check, e.g., inresponse to determining that the tape has not been written to in morethan a predetermined amount of time, the detection of errors whenreading, etc.

Conventional operations are typically performed in addition to the stepsbelow, including loading the tape in the tape drive, mounting the tape,processing index information about the data on the tape, etc.

Operation 1902 includes loading an initialized cartridge into a drive.Decision 1904 determines whether a health check has been requested forthis cartridge. If not, the cartridge is deemed ready for read and/orwrite operations. See operation 1906. If a health check has beenrequested, the SBD is measured at multiple locations along tape usingconstant tension in operation 1908. Preferably, the tension is about thesame as the tension used during initialization. In operation 1910, themeasurements are compensated for any of a variety of parameters. Forexample, the measurements may be compensated for the actual headdimensions, as determined using the drive information. Depending on thetype of health check desired, it may be advantageous to also compensatefor temperature and/or humidity. In operation 1912, the currentcharacterization of tape SBD is compared with the reference values inthe CM of the cartridge that were created during initialization. Theeffects of aging, such as the extent of creep, may be characterizedbased on the comparisons, which are preferably done for each SBDmeasurement, but may be performed for a subset thereof. Any type ofindication or value for such characterization may be used. An alert maybe output to indicate such things as that the comparison indicates thepotential for data loss and the user should consider migrating, orshould migrate, the data.

In the example shown, if the absolute value of the difference betweenthe reference value in the CM and the current SBD measurement is in arange, such as below a value X, then the creep may be characterized asminimal (e.g., Green Status). See operation 1914. If the absolute valueof the difference between the reference value in the CM and the currentSBD measurement is in another range, such as above a value X, then thecreep may be characterized as potentially affecting read/writeoperations (e.g., Yellow Status). See operation 1916. If the absolutevalue of the difference between the reference value in the CM and thecurrent SBD measurement is in a third range, such as above a value X bya predefined amount, then the creep may be characterized as extremelylikely to affect read/write operations (e.g., Red Status). See operation1918. Actions such as those mentioned above may be taken based on theStatus.

This process 1900 enables, among other things, characterization of theaging of a tape so that remedial measures may be taken if desired,thereby minimizing the likelihood of data loss. For example, if thecartridge is assigned a Red Status, the data may be moved to a differentcartridge. Once the data is moved and the tape can be erased, thecartridge can be re-initialized and used as normal. The creep no longeris considered an issue because the tape is characterized in its presentstate.

The present invention may be a system, a method, and/or a computerprogram product. The computer program product may include a computerreadable storage medium (or media) having computer readable programinstructions thereon for causing a processor to carry out aspects of thepresent invention.

The computer readable storage medium can be a tangible device that canretain and store instructions for use by an instruction executiondevice. The computer readable storage medium may be, for example, but isnot limited to, an electronic storage device, a magnetic storage device,an optical storage device, an electromagnetic storage device, asemiconductor storage device, or any suitable combination of theforegoing. A non-exhaustive list of more specific examples of thecomputer readable storage medium includes the following: a portablecomputer diskette, a hard disk, a random access memory (RAM), a ROM, anerasable programmable read-only memory (EPROM or Flash memory), a staticrandom access memory (SRAM), a portable compact disc read-only memory(CD-ROM), a digital versatile disk (DVD), a memory stick, a floppy disk,a mechanically encoded device such as punch-cards or raised structuresin a groove having instructions recorded thereon, and any suitablecombination of the foregoing. A computer readable storage medium, asused herein, is not to be construed as being transitory signals per se,such as radio waves or other freely propagating electromagnetic waves,electromagnetic waves propagating through a waveguide or othertransmission media (e.g., light pulses passing through a fiber-opticcable), or electrical signals transmitted through a wire.

Computer readable program instructions described herein can bedownloaded to respective computing/processing devices from a computerreadable storage medium or to an external computer or external storagedevice via a network, for example, the Internet, a local area network, awide area network and/or a wireless network. The network may comprisecopper transmission cables, optical transmission fibers, wirelesstransmission, routers, firewalls, switches, gateway computers and/oredge servers. A network adapter card or network interface in eachcomputing/processing device receives computer readable programinstructions from the network and forwards the computer readable programinstructions for storage in a computer readable storage medium withinthe respective computing/processing device.

Computer readable program instructions for carrying out operations ofthe present invention may be assembler instructions,instruction-set-architecture (ISA) instructions, machine instructions,machine dependent instructions, microcode, firmware instructions,state-setting data, or either source code or object code written in anycombination of one or more programming languages, including an objectoriented programming language such as Smalltalk, C++ or the like, andconventional procedural programming languages, such as the “C”programming language or similar programming languages. The computerreadable program instructions may execute entirely on the user'scomputer, partly on the user's computer, as a stand-alone softwarepackage, partly on the user's computer and partly on a remote computeror entirely on the remote computer or server. In the latter scenario,the remote computer may be connected to the user's computer through anytype of network, including a local area network (LAN) or a wide areanetwork (WAN), or the connection may be made to an external computer(for example, through the Internet using an Internet Service Provider).In some embodiments, electronic circuitry including, for example,programmable logic circuitry, field-programmable gate arrays (FPGA), orprogrammable logic arrays (PLA) may execute the computer readableprogram instructions by utilizing state information of the computerreadable program instructions to personalize the electronic circuitry,in order to perform aspects of the present invention.

Aspects of the present invention are described herein with reference toflowchart illustrations and/or block diagrams of methods, apparatus(systems), and computer program products according to embodiments of theinvention. It will be understood that each block of the flowchartillustrations and/or block diagrams, and combinations of blocks in theflowchart illustrations and/or block diagrams, can be implemented bycomputer readable program instructions.

These computer readable program instructions may be provided to aprocessor of a general purpose computer, special purpose computer, orother programmable data processing apparatus to produce a machine, suchthat the instructions, which execute via the processor of the computeror other programmable data processing apparatus, create means forimplementing the functions/acts specified in the flowchart and/or blockdiagram block or blocks. These computer readable program instructionsmay also be stored in a computer readable storage medium that can directa computer, a programmable data processing apparatus, and/or otherdevices to function in a particular manner, such that the computerreadable storage medium having instructions stored therein comprises anarticle of manufacture including instructions which implement aspects ofthe function/act specified in the flowchart and/or block diagram blockor blocks.

The computer readable program instructions may also be loaded onto acomputer, other programmable data processing apparatus, or other deviceto cause a series of operational steps to be performed on the computer,other programmable apparatus or other device to produce a computerimplemented process, such that the instructions which execute on thecomputer, other programmable apparatus, or other device implement thefunctions/acts specified in the flowchart and/or block diagram block orblocks.

The flowchart and block diagrams in the Figures illustrate thearchitecture, functionality, and operation of possible implementationsof systems, methods, and computer program products according to variousembodiments of the present invention. In this regard, each block in theflowchart or block diagrams may represent a module, segment, or portionof instructions, which comprises one or more executable instructions forimplementing the specified logical function(s). In some alternativeimplementations, the functions noted in the block may occur out of theorder noted in the figures. For example, two blocks shown in successionmay, in fact, be executed substantially concurrently, or the blocks maysometimes be executed in the reverse order, depending upon thefunctionality involved. It will also be noted that each block of theblock diagrams and/or flowchart illustration, and combinations of blocksin the block diagrams and/or flowchart illustration, can be implementedby special purpose hardware-based systems that perform the specifiedfunctions or acts or carry out combinations of special purpose hardwareand computer instructions.

Moreover, a system according to various embodiments may include aprocessor and logic integrated with and/or executable by the processor,the logic being configured to perform one or more of the process stepsrecited herein. By integrated with, what is meant is that the processorhas logic embedded therewith as hardware logic, such as an applicationspecific integrated circuit (ASIC), a field programmable gate array(FPGA), etc. By executable by the processor, what is meant is that thelogic is hardware logic; software logic such as firmware, part of anoperating system, part of an application program; etc., or somecombination of hardware and software logic that is accessible by theprocessor and configured to cause the processor to perform somefunctionality upon execution by the processor. Software logic may bestored on local and/or remote memory of any memory type, as known in theart. Any processor known in the art may be used, such as a softwareprocessor module and/or a hardware processor such as an ASIC, a FPGA, acentral processing unit (CPU), an integrated circuit (IC), etc.

It will be clear that the various features of the foregoing systemsand/or methodologies may be combined in any way, creating a plurality ofcombinations from the descriptions presented above.

It will be further appreciated that embodiments of the present inventionmay be provided in the form of a service deployed on behalf of acustomer.

The inventive concepts disclosed herein have been presented by way ofexample to illustrate the myriad features thereof in a plurality ofillustrative scenarios, embodiments, and/or implementations. It shouldbe appreciated that the concepts generally disclosed are to beconsidered as modular, and may be implemented in any combination,permutation, or synthesis thereof. In addition, any modification,alteration, or equivalent of the presently disclosed features,functions, and concepts that would be appreciated by a person havingordinary skill in the art upon reading the instant descriptions shouldalso be considered within the scope of this disclosure.

While various embodiments have been described above, it should beunderstood that they have been presented by way of example only, and notlimitation. Thus, the breadth and scope of an embodiment of the presentinvention should not be limited by any of the above-described exemplaryembodiments, but should be defined only in accordance with the followingclaims and their equivalents.

What is claimed is:
 1. An apparatus, comprising: a magnetic head havingservo readers of known pitch; a drive mechanism for passing a magneticrecording tape over the magnetic head; and a controller electricallycoupled to the magnetic head, the controller being configured to:measure, using signals from the magnetic head, a servo band differenceat various locations along a length of the magnetic recording tape; andcause storage of servo band difference information about the servo banddifference measurements in association with the magnetic recording tape.2. An apparatus as recited in claim 1, wherein the controller isconfigured to take servo band difference measurements at the variouslocations along about an entire length of the magnetic recording tape.3. An apparatus as recited in claim 1, wherein the controller isconfigured to take at least 100 of the servo band differencemeasurements.
 4. An apparatus as recited in claim 1, wherein thecontroller is configured to maintain the magnetic recording tape atabout constant tension while measuring the servo band differences.
 5. Anapparatus as recited in claim 1, wherein the controller is configured tostore the servo band difference measurements and/or derivatives thereofon a tape cartridge housing the magnetic recording tape.
 6. An apparatusas recited in claim 1, wherein the known pitch is particular to themagnetic head, wherein the servo band difference measurements and/orderivatives thereof are compensated using the known pitch.
 7. Anapparatus as recited in claim 1, wherein the controller is configured tocontrol writing to the magnetic recording tape by: retrieving the servoband difference information associated with the magnetic recording tape;measuring, using signals from servo readers of the magnetic head, aservo band difference at various locations along a length of themagnetic recording tape; comparing the servo band differencemeasurements and/or derivatives thereof to the retrieved servo banddifference information; and controlling a writing operation based atleast in part on a result of the comparing.
 8. An apparatus as recitedin claim 7, wherein the measuring and comparing are performed during thewriting operation, wherein the writing operation is suspended inresponse to a result of the comparing being indicative of potentialoff-track writing.
 9. An apparatus as recited in claim 7, wherein thecontroller is configured to perform the measuring and comparing duringthe writing operation, and wherein the controller is configured toadjust an operating condition in response to the comparing for reducingoccurrence of off-track writing.
 10. An apparatus as recited in claim 9,wherein adjusting the operating condition includes adjusting a tapetension during the writing.
 11. An apparatus as recited in claim 9,wherein adjusting the operating condition includes adjusting pitchesbetween transducers of the magnetic head during the writing.
 12. Anapparatus as recited in claim 7, wherein the controller is configured toperform the measuring and comparing before the writing operation,wherein the controller is configured to suspend the writing operation inresponse to a result of the comparing being indicative of potentialoff-track writing.
 13. An apparatus as recited in claim 7, wherein thecontroller is configured to perform the measuring during the writingoperation, and wherein the controller is configured to store the servoband difference measurements and/or derivatives thereof on a tapecartridge housing the magnetic recording tape.
 14. An apparatus asrecited in claim 1, wherein the controller is configured to controlreading from the magnetic recording tape by: retrieving the servo banddifference information about the magnetic recording tape; measuring,using signals from servo readers of the magnetic head, a servo banddifference at various locations along a length of the magnetic recordingtape; comparing the servo band difference measurements and/orderivatives thereof to the retrieved servo band difference information;and control a reading operation based at least in part on a result ofthe comparing.
 15. An apparatus as recited in claim 14, wherein thecontroller is configured to perform the measuring and comparing duringthe reading operation, and wherein the controller is configured toadjust an operating condition in response to the comparing for reducingoccurrence of off-track reading.
 16. An apparatus as recited in claim15, wherein adjusting the operating condition includes adjusting a tapetension during the reading.
 17. An apparatus as recited in claim 15,wherein adjusting the operating condition includes adjusting pitchesbetween transducers of the magnetic head during the reading.
 18. Anapparatus as recited in claim 15, wherein adjusting the operatingcondition includes performing error recovery in response to a result ofthe comparing being indicative of off-track reading.
 19. An apparatus asrecited in claim 14, wherein the servo band difference information isindicative of a condition of the magnetic recording tape when the datacurrently being read was written.
 20. An apparatus as recited in claim14, wherein the servo band difference information is indicative of acondition of the magnetic recording tape before the data currently beingread was written.